The use of organic polyisocyanate adhesives is well known for the production of consolidated wood composites such as chipboard (OSB), fiberboard, and related composite wood products. Polyisocyanate adhesives are also known for use in making engineered lumber composites such as I-beams (I-joists), laminated veneer lumber (LVL), and other types of structural lumber composites. The cure of the polyisocyanate in these end use applications is believed to be facilitated by the moisture present in the wood substrate. In commodity wood composite boards, such as OSB, heated presses are used to accelerate the cure of the polyisocyanate adhesive. Press temperatures exceeding 100° C., and often above 200° C., are used to facilitate rapid cure in these applications. However, in specialized structural (or engineered) lumber applications it is often impractical to use heat to drive the cure of the adhesive because the engineered wood composite structures are too massive for adequate heat transfer. In these highly specialized structural applications, it is more preferred to use polyisocyanate adhesives that cure at ambient temperatures. The formulation of these “cold cure” polyisocyanate adhesives is also well known in the art. The challenge in formulating such resins is to achieve an adequate balance between the need for rapid cure at room temperature and a suitably long working time (or pot life).
It is known in the prior art to use low molecular weight tertiary amine catalysts to promote the moisture activated curing of polyisocyanate adhesives. Unfortunately, the use of these low molecular weight tertiary amine species, even those that are free of active hydrogen groups reactive towards isocyanates, has not proven to be entirely satisfactory because such amines are usually unstable in polyisocyanates of the type used most widely in the wood composites industry and they typically do not provide a long enough open time (working time) to allow for easy handling of the adhesive or adequate time for the assembly of adhesive joints.
Another class of catalyzed polyisocyanate wood adhesive systems suitable for use in one-component moisture curing applications that is described in the prior art includes cold-curing polyisocyanate wood adhesives that comprise isocyanate group terminated prepolymers of amine alkoxylates (see e.g. U.S. Pat. No. 6,368,714). These prepolymer compositions have been found to offer an excellent combination of long working time (long pot life) on wood substrates and good moisture curing characteristics under ambient temperature conditions (cold cure). However, these adhesive systems, when used in laminated lumber applications, have been shown, under cold cure conditions to cause excessive delamination and unsatisfactory performance when tested according to ASTM D-2559-00. It is postulated that the delamination problem may be caused by a slow cure and/or an incomplete cure of the adhesive.
Therefore, there is a strong need for an improved class of polyisocyanate-based cold curing wood adhesives that have both long pot life and excellent stability, while providing increased resistance to delamination, especially when tested according to the procedure of ASTM D-2559-00, which is fully incorporated herein by reference.